Band pass circuit



July 31, 1934. T. J, wEYERs 1,968,237

BAND PASS CIRCUIT Filed Oct. l, 1931 INVENTOR THEoDovus J. wEYERa BY WWATTORNEY "'50 will be 450.

Patented July 31, 1934 ED ST FFCE BAND Pass CIRCUIT Application ctober1, 1931, Serial No. 566,22 ln Germany April 25, 1931 6 Claims.

The present invention relates to installations in which use is made offilters, as for example, in line telephony flow and high frequency),receiv ing installations for wireless telephony, television,

5 or the like.

The filter serves to obtain a uniform amplication of a determined bandof frequencies, whereas the frequencies lying outside this band aresubstantially not amplified. For this purpose use is made of filtersconsisting of a combination of one or more self-inductances and ca-'pacities Fig. 1 of the drawing represents, Vby way of example, oneembodiment, in which the lter consists of the circuits L1 C1, L2 C2 LaC3, L4

C4, L5 C5. Such a filter has five resonance frequencies. When neglectingthe damping due to the dissipation in the filter elements, thecharacteristic curve of this filter may be represented by the curve l ofFig. 2 in which the resonance frequencies are denoted by A, B, C, D andE.

The difference between these frequencies, and

therefore the width of the frequency band passed on by the filter, isdetermined by the ratio of L and C in each of the circuits. Due to thedissipation in the filter elements there are produced in thecharacteristic curve deviations which for the side frequencies aregreater than for the middle frequencies so that a characteristic' curveis obtained which has the shape of the curve 2 in Fig. 2.

The present invention has for its object to give this curve a shapewhich comes nearer to the ideal shape. Use is made for this purpose ofthe e fact that the phase of the energy in the output circuit relativeto that in the input circuit de- 35 pends on the frequency. According tothe invention, coupling back to the input side takes place from a pointof the filter which is so chosen that for the mean proportional betweenthe limit frequencies of the frequency band to be passed on, the energycoupled back is in counterphase with the energy directly supplied.

For the explanation thereof reference may be made to Fig. 2. If for thefrequency C the phase in the output circuit is equal to that in theinput circuit, the phase displacement for D will be 180 for E it will be360, and for an infinitely high frequency it would be 450. The phasedisplacement for the frequency B is 130; for a frequency A it will be360 and for a frequency 0 it When coupling back negatively, i. e., witha phase displacement of 180, from the output impedance this implies forthe frequency C a decrease of the amplication, for the frequencies B andD an increase of the amplification, and for A and E again a decrease. Asa result there is consequently obtained a iiattening of thecharacteristic curve for'the frequencies to be transmitted while thesides of the curve become steeper and the frequencies A and E are lessamplified. The curve S in Fig. 2 represents 60 the characteristic curveof the filter with retroaction. By coupling back still stronger there isobtained the curve 4.

The desired feed back action may be obtained with a circuit'arrangementas shown in Fig. 3 e5 which comprises a resistance R1, a filter composedof units L1 C1, L2v C2, L3 C3, L4 C4, L5 C5, and a resistance R2includedin the output circuit of an amplifier tube V1. The resistancesR1 and R2 are so chosen that the valueof the resistance R2, and also thetotal amount of the value of the resistance R1, and the internalresistance of the amplifier V1 are equal to the wave resistance of thefilter. A suitably chosen point of the resistance Rz is connected to theinput circuit in crder to obtain the required retroaction. The amplifiedenergy is preferably taken between the points A andB of the resistanceR2, consequently from that portion of the output impedance which is notcoupled back, said energy being supplied 30 through a transformer Ttothe input electrodes of a tube V2.

If, for exampledue 'to parasitic capacities, the filter is not perfectlysymmetrical so that not the phase displacement vof thepoint C but, forexample, that of the pointF would be equal to zero, the resistance R2 isreplaced by an impedance causing such a phase displacement that bycoupling back the above described required effect is obtained again. Forthis purpose a capacitive, or inductive, impedance may be connected inseries, or in parallel, with R2, or with part of R2. It will be evidentthat the invention is neither restricted to a filter of the abovedescribed type nor to the use of a filter consisting of five tunedcircuits. Use may be made of a filter of arbitrary construction, if onlycare is taken to provide the retroaction in such manner that thefrequency band to be passed on is aml plified more linearly, and thatthe frequencies lying outside this band are less amplified. If use ismade of a filter consisting of a large number of elements it isadvantageous, instead of couplingback over the whole filter, to coupleback over part thereof owing to which the number of amplications orattenuations is reduced.

While I have indicated and described one arrangement for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the par- CF; in,

ticular organization shown and described, but that many modificationsmay be made without departing from the scope of my invention as setforth in the appended claims'.

What is claimed is:

1. A band filter, adapted to be connected between a source of highfrequency energy of a predetermined frequency band width and a loadcircuit, comprising a plurality of resonant networks, each network beingresonant to a different frequency component of said band, and a pathconnected between a predetermined point in the output side of the iterand the input side for feeding back said energy, said pointbeing sochosen that for the mean proportional between the limiting frequenciesof the said band the energy fed back is in phase opposition to theenergy impressed on said input side from said source.

2. A band filter, adapted to be connected between a source of highfrequency energy of `a pre'- determined frequency band width and a loadcircuit,'comprising atleast 'five resonant networks, each Vnetworkbeingr resonant to a different fre,- quency component of said band, anda path connected'between a predetermined point in the output side of thefilterand the input side for feeding back said energy, said point beingso chosen that 'for the mean proportional between the limitingfrequencies of the said 'band the energy fed back is in phase oppositionto the energy impressed on said input side from said source.

3. A'band filter, adapted to be connected between a source of highfrequency energy of a predetermined frequency band width and a loadcirthatfor the'mean proportional between the limiting frequencies of thesaid band the energy fed vback 'is in phase opposition tothe energyimressed on said input side vfrom said source, said -filter inputincluding a tube having its input electrodes connected to said path, andan additional tube coupled-to the output side of the said filter.

4. A band filter, adapted to be connected be- `tween a'sourceof highfrequency energy of a predetermined frequency band width'and a loadcircuit, comprising a plurality of resonant networks,

resistor.

5. The combination with a source of high frequency energy wherein theenergy comprises a band of frequencies, an amplifier tube having itsinput coupled to the source, and a second tube, of a band filter havingits input terminals coupled to the output electrodes of the amplier tubeand its output terminals coupled to the input of thesecondtube, saidfilter including a plurality of resonant networks, each of the networksbeing resonant to a diiferent one of said frequencies, and means forfeeding back a portion of the energy output Vof the filter to theamplifier tube input, the energy fed back 'being in phase oppositionwith the energy fed to the .amplierinput from said source whereby energyof frequencies lying outside said band'is substantially not transmittedthrough the filter while a substantially uniform transmission of thesaid band frequencies is secured.

6. The combination with a source of high frequency energy wherein theenergy comprises a band of frequencies, an amplier tube having its inputcoupled to the source and a second tube, of a band 'lter having itsinput terminals coupled to the output electrodes of the ampliiiertubeand its output terminals coupled to the input of the second tube, saidfilter including a plurality of alternate series and shunt resonantnetworks, each of the networks being resonant to a diffe-rent one ofsaid frequencies, and means for feeding 'back aportion of the energyoutput of the filter to the amplifier tube input, the energy fed'backbeing in phase opposition with the energy fed to the amplifier inputfrom said source whereby energy of frequencies Vlying outside said bandis substantially not transmitted through the 'lter 'while a'substantially uniform transmission of the said band frequencies issecured. n

V'II-IEODORUS JOSEPI-IUS WEYERS.

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